Micron-scale fabrication has enabled the development of micro-optical elements that are used in a variety of optical-electronic applications. A micro-optical element (MOE) offers compact, light-weight optics that can be mass-produced using low cost replication techniques. With the given trend towards miniaturization, these features are highly attractive. MOEs may be refractive, such as microlenses and microlens arrays, and bend or focus light according to geometric optics. MOEs may be diffractive, such as phase plates, diffraction gratings, diffractive lenses, and so forth, and alter light based on Fourier optics. MOEs may also be mixed refractive/diffractive lenses which typically involves the refractive lenses having a surface textured with diffracting patterns.
MOEs include three-dimensional surface structures that are typically based on complex mathematical modeling. Dimensional accuracy and fabrication cost are important factors in production. MOEs can be divided into two basic families: continuous relief and binary, or multi-level, micro-optics. Continuous relief microstructures have a smoothly varying surface profile between multiple steps. Fabrication methods for continuous-relief micro-optics include direct writing, such as by laser beam or e-beam, direct machining, photoresist reflow, and gray tone lithography. Binary optical elements have phase levels with a number of steps and a flat surface of a constant height between the steps. A common fabrication method for binary MOEs is a high-resolution mask lithography and etching process.
MOEs are still an emerging technology that has not yet developed a uniformly accepted nomenclature. The following components are subgroups of micro-optical elements and include, Diffractive Optical Element (DOE), Binary Optical Element (BOE), Binary Optic, Microstructured Optic, grating, blazed grating, fresnel element, micro-relief element, nanoperiodic surface structures, refractive micro optics, subwavelength structure, and subwavelength structured surface. As can be expected, each subgroup has certain features specific to that group. As used herein, the term MOE indicates all of the above listed components. It is also expected that there will be applications where MOEs will be integrated on top of light detecting and light emitting devices and integrated circuits.
DOEs are a broad class of optical components. Unlike conventional optical components utilizing refraction and/or reflection, DOEs utilize the wave nature of light and rely on amplitude, phase, and the polarization state of light. With diffractive optics all these properties can be modified using nano/micro structures. Diffraction structures on a surface can be of several principles including binary, multi-level, continuous profile, index modulated, and holographica.
All MOEs, either refractive or diffractive, have in common that the wave nature of light is applied for their design and their performance and tolerance modelling. Furthermore, all MOEs used herein have three-dimensional surface structures with a dimension of about 0.01 microns to about 10 micron.
A MOE may employ a film or coating disposed on its surface structures for a variety of purposes. The film or coating may be a semiconductor layer, insulator, metal contact, passivation layer, anti-reflective coating, optical filter, waveguide or other coating. The film or coating preferably follows a surface pattern of the MOE. Otherwise, the functionality may be severely degraded or the element may be out of a specification range. In particular, MOEs with structures having dimensions smaller than the wavelength of light are difficult structures on which to apply conformal coatings of uniform thickness. Such a structure is commonly referred to herein as a subwavelength structure. Fabrication is further complicated where such structures have high aspect ratios.
A number of techniques exist to provide thin films and coatings. These techniques include sputtering, evaporation, pulsed laser ablation, oxidation, chemical vapor deposition, electroplating, and other techniques commonly known in the art. However, these conventional techniques are not able to provide conformal coatings with uniform thickness for a subwavelength structure.
Referring to FIG. 1, a cross-section view of a substrate 10 is shown wherein a thin film 12 is deposited by physical vapor deposition (PVD). The PVD technique removes coating material from a source using high temperature (evaporation) and/or bombards the surface with highly energetic particles (sputtering). Removed material particles have kinetic energy and this kinetic energy is used to transfer coating material onto the substrate 10. On an atomic scale, sputtered atoms tend to travel in straight lines without in-flight collisions from a cathode to the sample.
When coating material arrives on a substrate its energy does not allow extended movement on the substrate surface. As shown, the thin film 12 does extend to a shadowed region 14 and does not provide step coverage. This problem is pronounced where a subwavelength structure is involved. The shadowed region 14 may be caused by the shadow of the substrate 10 or even by the growing thin film 12.
Methods to improve conformity involve rotating the substrate 10 or heating the substrate 10 to increase atom mobility. However, the shadowed region will not be fully eliminated. Furthermore, because PVD is based on flying material there is always a “line of sight” problem so that sides of walls are difficult to coat. In case of high aspect ratio good step coverage is impossible.
Referring to FIG. 2, a cross-sectional view is shown of a structure 20 is shown having a trench 22. A thin film 24 is deposited by chemical vapor deposition (CVD). CVD techniques use continuous precursor flow to mix precursors in a reaction chamber where a structure is placed. Energy is applied to the structure and the precursor vapor to form a layer of a desired composition.
CVD methods have difficulties in applying very thin films because the film does not always conform. As shown in FIG. 2, the growth of the thin film 24 is not always uniform and does not exactly follow the underlying surface. Voids 26 are created underneath the film 26 as the trench 22 is filled. Thus, the reliability of the CVD method for thin films is often in question.
Referring to FIG. 3, a similar problem is shown wherein a structure 30 includes a narrow trench 32. A thin film 34 is deposited by a technique such as by CVD in order to completely fill the trench 32. The thin film 34 pinches off at the opening 36 of the trench 32 before the trench 32 is completely filled. This creates a void 38 within the trench 32 which destroys functionality of the structure 30.
Referring to FIG. 4, another structure 40 is shown having multiple step levels. A thin film 42 is deposited by sputtering. Directionality of the sputtering is indicated by the arrows 44. Directionality may be achieved by long distance origination and mask and/or with ion beam sputtering. No side wall coating exists where the film 42 is less than a step height.
Referring to FIG. 5, a structure 50 is shown similar to that of FIG. 4. The sidewalls 52 are completely covered by increasing the thickness of the film 54. This is, however, practical only in applications having shallow steps and low aspect ratios.
Referring to FIG. 6, a structure 60 is shown with a trench 62 having a relatively high aspect ratio. Difficulties arise with conventional techniques, such as CVD, in having a film 64 conform to the deep trench 62. As shown, a void 66 occurs when the film 64 fails to fully conform to the trench 62.
Referring to FIG. 7, a structure 70 is shown with a thin film 72 that is deposited by CVD or sputtering. The film 72 conforms poorly to the structure 70 and provides poor structural dimensions.
The present inventors have recognized that the problems with thin film depositions illustrated in FIGS. 1 to 7 often become even more pronounced with subwavelength structures. Techniques for overcoming void formations and providing controlled conformal coatings would greatly improve the performance of micro-optical elements. Such techniques would have particular application beneficial for light transmissive optics that include anti-reflective coatings.